Electrophotographic marking is a well known and commonly used method of copying or printing original documents. Electrophotographic marking is typically performed by exposing a light image of an original document onto a substantially uniformly charged photoreceptor. In response to that light image the photoreceptor discharges so as to create an electrostatic latent image of the original document on the photoreceptor's surface. Toner particles are then deposited onto the latent image so as to form a toner powder image. That toner powder image is then transferred from the photoreceptor, either directly or after an intermediate transfer step, onto a substrate such as a sheet of paper. The transferred toner powder image is then fused to the substrate using heat and/or pressure. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for the creation of another image.
The foregoing generally describes a typical black and white electrophotographic printing machine. Electrophotographic printing can also produce color images by repeating the above process for each color of toner that is used to make the color image. For example, the charged photoconductive surface may be exposed to a light image which represents a first color, say cyan. The resultant electrostatic latent image can then be developed with cyan toner particles to produce a cyan toner image which is subsequently transferred and fused onto a substrate. The process can then be repeated for a second color, say magenta, then a third color, say yellow, and finally a fourth color, say black. If the toner particles are placed in a superimposed registration the desired composite color image is formed on the substrate.
The color printing process described above superimposes the various color toner powder images directly onto a substrate. Another electrophotographic color printing process uses an intermediate transfer member. In systems which use an intermediate transfer member successive toner images are transferred in superimposed registration from the photoreceptor onto the intermediate transfer member. Only after the composite toner image is formed on the intermediate transfer member is that image transferred and fused onto the substrate.
The most common developing materials are dry powder toners. Dry powder developers are typically comprised of not only toner particles but also of carrier granules. The toner particles triboelectrically adhere to the carrier granules until the toner particles are attracted onto the latent image. An alternative to dry powder developing materials are liquid developers.
Liquid developers, also referred to as liquid inks, have a liquid carrier into which toner particles are dispersed. When developing with liquid developers both the toner particles and the liquid carrier are advanced into contact with the electrostatic latent image. The liquid carrier is then removed by blotting, evaporation, or by some other means, leaving the toner particles behind.
Intermediate transfer members can also be used in the fusing process. Intermediate transfer members which are used in fusing are referred to herein as transfusing members, and the combined processes of transferring and fusing is called transfusing. Transfusing is highly desirable since the size and cost of transfusing printing machines can be less than comparable printing machines which have separate transfer and fusing stations. Other advantages such as improved image quality can also be obtained by transfusing. Transfusing members are usually pinched between one or more contact rollers and a backup roller such that a fusing pressure is created between the nip of the backup roller and the transfusing member. During fusing a substrate passes between the backup roller and the transfusing member and heat is applied to the toner image. The combination of heat and pressure causes the toner image to fuse onto the substrate. Transfusing may be done without heat, but the resulting quality is usually inferior.
One potential problem with transfusing members is that the transfusing member needs to be hot to provide high-quality fusing. That heat can damage the photoreceptor and can interfere with the transfer process.
Additionally, it is desirable that the power consumed during fusing be low. Meeting the conflicting requirements of fusing without damaging the photoreceptor while using little power is difficult. Thus, energy efficient transfusing stations, and printing machines which use such transfusing stations, which reduce the heating of the photoreceptor are highly desirable.
Various approaches have been devised to produce multicolor color copies. The following U.S. patents may be useful references:
U.S. Pat. No. 3,392,667 PA0 Patentee: Cassel et al. PA0 Issued: Jul. 16, 1968 PA0 U.S. Pat. No. 3,399,611 PA0 Patentee: Lusher PA0 Issued: Sep. 3, 1968 PA0 U.S. Pat. No. 3,955,530 PA0 Patentee: Knechtel PA0 Issued: May 11, 1976 PA0 U.S. Pat. No. 3,957,367 PA0 Patentee: Goel PA0 Issued: May 18, 1976 PA0 U.S. Pat. No. 4,348,098 PA0 Patentee: Koizumi PA0 Issued: Sep. 7, 1982 PA0 U.S. Pat. No. 4,515,460 PA0 Patentee: Knechtel PA0 Issued: May 7, 1985 PA0 U.S. Pat. No. 4,588,279 PA0 Patentee: Fukuchi et al. PA0 Issued: May 13, 1986 PA0 U.S. Pat. No. 4,935,788 PA0 Patentee: Fantuzzo et al PA0 Issued Jun. 19, 1990 PA0 U.S. Pat. No. 5,254,424 PA0 Patentee: Felder PA0 Issued: Oct. 19, 1993 PA0 U.S. Pat. No. 5,352,558 PA0 Patentee: Simms et al PA0 Issued: Oct. 4, 1994 PA0 U.S. Pat. No. 5,355,201 PA0 Patentee: Hwang PA0 Issued: Oct. 11, 1994
The disclosures of the above-identified patents may be briefly summarized as follows:
U.S. Pat. No. 3,392,667 discloses a plurality of print cylinders having gravure engravings on their peripheries. Powder feed hoppers having rotating brushes apply powder to the print cylinders. The powder images from the print cylinders are transferred to an offset roller in superimposed registration with one another. The resultant powder image is then transferred from the offset roller to paper or sheeting.
U.S. Pat. No. 3,399,611 describes four image transfer stations disposed about the periphery of a rotatable cylindrical metal drum. Each image transfer station is basically the same and includes a photoconductive drum charged by a charging wire and then rotated into alignment with an image exposure station to record a latent image thereon. Powder particles are then cascaded across the latent image to develop it. The powder image is then transferred to the surface of the metal drum. The powder particles are of different colors. The completed powder image is transferred from the metal drum to an article to be decorated.
U.S. Pat. No. 3,955,530 discloses a color image forming electrophotographic printing machine. Different color developers are used to develop the latent images recorded on the photoconductive drum. Each developed image is sequentially transferred to an intermediate transfer drum. A cleaning blade is used to clean the photoconductive drum between developing different color developers. The complete image is transferred from the intermediate drum to a copy sheet.
U.S. Pat. No. 3,957,367 describes a color electrophotographic printing machine in which successive different color toner powder images are transferred from a photoconductive drum to an intermediate roller, in superimposed registration with one another, to an intermediary roller. The multi-layered toner powder image is fused on the intermediary roller and transferred to the copy sheet.
U.S. Pat. No. 4,348,098 discloses an electrophotographic copying apparatus which uses a transfix system. In a transfix system, the developed image is transferred from the photoconductive member to an intermediate roller. The intermediate roller defines a nip with a fixing roller through which the copy sheet passes. The developed image is then transferred from the intermediate roller to a copy sheet. The developing unit of the copying apparatus may either be a dry or wet type.
U.S. Pat. No. 4,515,460 describes a color electrophotographic copying machine in which four developer units develop four latent images recorded on a photoconductive drum with different color toner particles. The different color toner powder images are transferred to an endless belt in superimposed registration with one another. The resultant toner powder image is then transferred from the belt to a copy sheet.
U.S. Pat. No. 4,588,279 discloses an intermediate transfer member that has a dry toner image transferred thereto from the surface of a toner image forming member. The toner image is then transferred from the transfer member to a recording paper.
U.S. Pat. No. 4,935,788 discloses a multicolor printing system that uses liquid developing and an intermediate member.
U.S. Pat. No. 5,254,424 discloses a liquid developer material which contains toner particles formed from a urethane modified polyester.
U.S. Pat. No. 5,352,558 discloses a liquid developer system which uses an absorbing belt.
U.S. Pat. No. 5,355,201 discloses an apparatus for developing an electrostatic latent image with liquid toner.